N-Heteropolycyclic compounds are promising organic n-channel semiconductors for applications in field effect transistors. The adsorption behavior of these molecules on inorganic substrates is of great interest, since it affects the transport properties. Utilizing high-resolution electron energyloss spectroscopy (HREELS) and density functional theory (DFT), we determined the adsorption geometry of three different N-heteropolycyclic molecules as a function of coverage on Au(111). All three π-conjugated aromatic molecules adopt a planar geometry with respect to the substrate in both the monolayer (ML) and thin films (up to 10 ML). Contrary, in their crystal structure the molecules are tilted up to 82°between the molecular planes in neighboring stacks. Electronic HREELS and DFT calculations allowed the determination of the optical gaps of the molecules which are unaffected by the nitrogen substitution of the polycyclic aromatic hydrocarbons, while the frontier orbitals of the N-heteropolycyclic compounds are stabilized. The present study provides important aspects such as adsorption and electronic properties which are essential for designing organic-molecules-based electronic devices.

N-Heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field effect transistors. Here, we studied the structural and the electronic properties of an arrow-shaped N-heteropentacene derivative (triisopropylsilyl-dibenzodiazapentacene, TIPS-BAP) adsorbed on Au(111) in the monolayer and thin films using temperature-programmed desorption, vibrational and electronic high-resolution electron energy loss spectroscopy, and two-photon photoemission spectroscopy. In addition, we performed state-of-the-art quantum chemical calculations to further elucidate electronic properties. TIPS-BAP adopted an adsorption geometry in which the molecular backbone is oriented parallel to the Au(111) surface. We quantitatively determined the energetic position of several unoccupied as well as occupied molecular electronic states (transport states) with respect to the Fermi level of the gold substrate and resolved the optical gap (S0 → S1 transition) to be 1.9 eV. Compared to the corresponding polycyclic aromatic hydrocarbon TIPS-dibenzodipentacene (TIPS-BP), the optical gap is reduced by 0.2 eV due to nitrogen substitution. Based on our quantum chemical calculations, we attributed this effect to a stabilization of the first excited singlet state (S1) in the polar environment. Furthermore, an intense α-band (S0 → S2) in TIPS-BAP compared to TIPS-BP is observed due to an enhanced oscillator strength in the N-heteropolycyclic aromatic compound.

Oligoacenes have recently received much attention because of their ability to undergo singlet fission, a process which may improve the energy conversion efficiency in organic solar cells. Here, we studied the structural and the electronic properties of triisopropylsilylethynyl-pentacene (TIPS-Pn) adsorbed on Au(111) from monolayer to multilayer coverages using multiexperimental techniques [temperature-programmed desorption, vibrational and electronic high-resolution electron energy loss spectroscopy, and energy- and time-resolved two-photon photoemission (2PPE)]. We demonstrated that well-defined TIPS-Pn molecular films can be prepared via evaporation. TIPS-Pn adopt on Au(111) a molecular adsorption geometry in which the pentacene backbone is oriented parallel to the substrate. We determined the energetic position of several unoccupied and occupied electronic states among others originating from the lowest unoccupied molecular orbital and the highest occupied molecular orbital. Femtosecond time-resolved 2PPE enabled us to resolve the time scales for the singlet fission process, which are coverage-dependent. The present study provides important parameters such as energetic positions of molecular electronic states as well as the electronically excited-state dynamics involved in the singlet fission process, which are essential for designing organic-molecule-based optoelectronic devices.

Efficient charge injection at organic semiconductor/metal interfaces is crucial for the performance of organic field effect transistors. Interfacial hybrid band formation between electronic states of the organic compound and the metal electrode facilitates effective charge injection. Here, we show that a long-range ordered monolayer of a flat-lying N-heteropolycyclic aromatic compound on Au(111) leads to dispersing occupied and unoccupied interfacial hybrid bands. Using angle-resolved two-photon photoemission we determine their energy level alignment and dispersion relations. We suggest that band formation proceeds via hybridization of a localized occupied molecular state with the d-bands of the Au substrate, where the large effective mass of the d-bands is significantly reduced in the hybrid band. Hybridization of an unoccupied molecular state with the Au sp-band leads to a band with an even smaller effective mass.

Surface-bound porphyrins are promising candidates for molecular switches, electronics and spintronics. Here, we studied the structural and the electronic properties of Fe-tetra-pyridil-porphyrin adsorbed on Au(111) in the monolayer regime. We combined scanning tunneling microscopy/spectroscopy, ultraviolet photoemission, and two-photon photoemission to determine the energy levels of the frontier molecular orbitals. We also resolved an excitonic state with a binding energy of 420 meV, which allowed us to compare the electronic transport gap with the optical gap.